General weight lifting is a good, but specifically Google rotator cuff exercises - stuff like internal and external rotations. Doing them now can really help prevent shoulder issues in the future.
To Mania's question: Depending on the source car accidents cause something like 15 to 17 deaths per 100,000 people. The overall rafting/kayaking/canoing death rate is something like 2-4 per 100,000. Rafting has a rate of .55 to .87 per 100,000 and kayaking is 2.9 per 100,000 (AW summation).
I posted a chart on boatertalk a while back with a bunch of other sports and causes of death but I can't find it now.
Check out this pdf, it has comparative accident and death rates as well as the most common injuries for both rafters and kayakers:
Overall most boating fatalities are due to alcohol and not wearing life jackets. But this is mainly the power boating and inner tubing crowd.
Rafters, canoeists, and to a limited extent kayakers, get into trouble when they are inexperienced and in over their heads. Kayakers who are really good and doing really hard stuff account for a large portion of kayaking deaths. The numbers don't show the same for rafters, but honestly that is probably in part because there aren't that many rafters running class V and V+ creeks. Also rafts don't tend to piton and trap people underwater.
The most common kayaking injury besides minor stuff like blisters and abrasions is shoulder dislocations.
Rafters most common injuries is from hitting something in the boat - like a t-grip or a cooler. Next up is hitting stuff while swimming. The pdf actually recommends that rafters wear face masks to avoid cuts and bruises. Maybe I should ask WRSI to sponsor me ; )